The present invention is related to angiography and, in particular, the invention relates to a system and method for producing time-resolved, three-dimensional (3D) angiographic images.
Since the introduction of angiography beginning with the direct carotid artery punctures of Moniz in 1927, there have been ongoing attempts to develop angiographic techniques that provide diagnostic images of the vasculature, while simultaneously reducing the invasiveness associated with the procedure. For decades, post-processing of images was largely limited to the use of film subtraction techniques. Initial angiographic techniques involved direct arterial punctures and the manipulation of a needle through which a contrast medium was injected. These practices were associated with a significant incidence of serious complications. The development of percutaneous techniques allowing the use of a single catheter to study multiple arterial segments reduced, but this by no means eliminated, these adverse events. In the late 1970's, a technique known as digital subtraction angiography (DSA) was developed based on real-time digital processing equipment. Because of the advantages of digital processing, it was originally hoped that DSA could be consistently implemented using an IV injection of contrast medium, thus reducing both the discomfort and the incidence of complications associated with direct IA injections.
However, it quickly became apparent that the IV-DSA technique was limited by problems due to suboptimal viewing angles and vessel overlap that could only be reduced by repeated injections. Even then, these factors were problematic unless a projection that avoided the overlap of relevant vascular structures could be defined. Similar problems occurred when using biplane acquisitions. Also, because of the limited amount of signal associated with the IV injection of contrast medium, IV-DSA was best performed in conditions with adequate cardiac output and minimal patient motion. IV-DSA was consequently replaced by techniques that combined similar digital processing with standard IA angiographic examinations. Nevertheless, because DSA can significantly reduce both the time necessary to perform an angiographic examination and the amount of contrast medium that was required, its availability resulted in a significant reduction in the adverse events associated with angiography. Due to steady advancements in both hardware and software, DSA can now provide exquisite depictions of the vasculature in both 2D and rotational 3D formats. Three-dimensional digital subtraction angiography (3D-DSA) has become an important component in the diagnosis and management of people with a large variety of central nervous system vascular diseases.
Current limitations in the temporal resolution capabilities of x-ray angiographic equipment require that rotational acquisitions be obtained over a minimum time of about 5 seconds. Even with perfect timing of an acquisition so that arterial structures are fully opacified at the onset of a rotation, there is almost always some filling of venous structures by the end of the rotation. Display of a “pure” image of arterial anatomy is only achieved by thresholding such that venous structures, which contain lower concentrations of contrast medium than arterial structures, are no longer apparent in the image. This limitation is a significant factor in making it prohibitively difficult to accurately measure the dimensions of both normal and abnormal vascular structures. Current DSA-based techniques do not depict the temporal sequence of filling in a reconstructed 3D-DSA volume.
In recent years, competition for traditional DSA has emerged in the form of CT angiography (CTA). Like traditional DSA, CTA relies upon ionizing radiation and, thus, presents a substantial drawback of requiring the subject to receive a dose of the ionizing radiation in order to acquire the desired images. Furthermore, while CTA provides high spatial resolution, it is not time-resolved unless the imaging volume is severely limited. CTA is also limited as a standalone diagnostic modality by artifacts caused by bone at the skull base and the contamination of arterial images with opacified venous structures. Further, CTA provides no functionality for guiding or monitoring minimally-invasive endovascular interventions.
Recently, improvements in DSA have been made that overcome many of the drawbacks presented by traditional DSA and newer imaging techniques, like CTA. Specifically, a technique referred to as 4D DSA has been developed for generating detailed series of time-resolved, three-dimensional medical images of a subject, with both high temporal resolution and excellent spatial resolution, by imparting temporal information from a time-series of 2D images into a still 3D image. To achieve this, 4D DSA techniques acquire a time-series of 2D-DSA images using a fluoroscopy system and acquire a 3D image substantially without temporal resolution using a the same or a different fluoroscopy or CT system. For example, in some cases, these two data sets may be acquired using a common acquisition performed, for example, using a C-arm CT system or may combine a C-arm or gantry-based CT system with a biplane fluoroscopy system to complete the acquisitions. A time-resolved, 3D image is produced by selectively combining the 3D image without temporal resolution and the time-series of 2D images. While such 4D DSA systems and methods improve upon traditional DSA or CTA capabilities, they require the use of ionizing radiation.
It would therefore be desirable to have a system and method for providing a time-resolved 3D image that is capable of providing the clinically-desirable information provided by DSA, 4D DSA, and CTA, but without the expense and complexity of combining multiple imaging sub-systems or the use of ionizing radiation.